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52 



THE AUEIFEROUS GRAVELS OF THE SIEREA NEVADA. 



on Butte, Chico, and Cow creeks, as also at Tuscan Springs, where the beds 

 are rich in fossils, which have been figured and described in the volumes of 

 the Palaeontology of California. 



The most interesting locality of this formation is that at Pence's Ranch, in 

 Butte County, about nine miles north of Oroville. This has been described 

 in Geology I. (p. 209), and need only be referred to here, as illustrating 

 excellently, not only the relations of the marine Tertiary and Cretaceous to 

 the underlying auriferous slate series, but also the position of the volcanic 

 formations of the west slope of the Sierra, which at the northern extremity 

 of the Great Valley have flowed down so far as to cover the marine sedimen- 

 tary beds, while in the counties farther south they have not usually reached 

 the foot-hills. At Pence's we see the Cretaceous strata resting nearly 

 horizontally on the rocks of the auriferous slate series, which latter dip at 

 a high angle towards the northeast. On the Cretaceous lie, nearly conform- 



ably, beds of Tertiary age, which are probably of fresh-water origin, over 

 these stratified masses of volcanic ashes and tufa, and, capping the whole, 

 heavy masses of solid basaltic lava, which has been eroded away, so as to 

 leave isolated patches with flat summits, forming the peculiar elevations 

 known in California as « Table Mountains/' The peculiarly interesting fea- 

 ture of this locality is, that here we have the volcanic and fresh-water detrital 

 formations of the Sierra — the especial subject of this volume — brought into 

 contact with the marine strata which are of more recent age than the au- 

 riferous slate series, and which, as has been described, form an almost con- 

 tinuous belt along the foot-hills of the Sierra. The relations of the different 

 sets of formations to each other and to the bed-rock series are so clearly 

 displayed at this locality, that there can be no mistaking their nature, and 

 in the succeeding chapter we shall have occasion to present a great number 

 of facts, by the aid of which we shall arrive at a still better understanding of 

 the phenomena here presented. 



